1. Field of the Invention
The present invention relates to a radio frequency tuner. Such a tuner may be used as a cable tuner for receiving modulated digital signals from a cable distribution network, for example, as a set top box for receiving television signals. Other applications include use for cable telephony and as a modem for data signals.
2. Description of the Prior Art
Known types of cable tuners may be of the single conversion or double conversion type. By way of illustration, FIG. 1 of the accompanying drawings illustrates a double conversion cable tuner for digital signals. The tuner has an input 1 for connection to a cable distribution network and connected to an automatic gain control (AGC) stage 2. The output of the stage 2 is connected to a first frequency changer 3 for performing up-conversion and comprising a mixer 4 and a local oscillator 5 controlled by a phase locked loop (PLL) synthesiser 6.
The output of the first frequency changer 3 is connected via a first intermediate frequency filter 7 to a second frequency changer 8 of the down-conversion type. The second frequency changer 8 comprises a mixer 9 and a local oscillator 10 controlled by a PLL synthesiser 11. The output of the second frequency changer 8 is supplied by a second intermediate frequency filter 12 to an intermediate frequency (IF) amplifier 13 whose output supplies an IF signal to the output 14 of the tuner.
FIG. 2 of the accompanying drawings illustrates a typical cable distribution system. The system comprises an optical fibre “backbone” 20 which supplies a plurality of head end distribution units 21. Each of the units 21 services a plurality of properties, such as houses and offices, as illustrated at 22. A cable 23 connects each property to its head end unit 21, which supplies the signals from the distribution network and which also supplies power to a modem 24 including a tuner of the type shown in FIG. 1. The modem 24 is continuously powered and supplies a telephony service by means of a conventional twisted pair 25 and a cable service for further modem applications such as television and internet as illustrated at 26.
The tuner is required to convert any selected channel from the cable distribution network to an intermediate frequency in the form of a signal having characteristics which are sufficient to ensure acceptable perceived performance. For example, the signal to noise plus intermodulation performance must be sufficient for acceptable television or data signals to be provided. The signals from the cable distribution network are supplied to the AGC stage 2 which controls the signal level supplied to the first frequency changer 3 so as to provide an acceptable intermodulation performance of the first frequency changer. In general, little or no filtering takes place before the frequency changer 3, which is therefore required to have a high level of performance.
The frequency changer 3 performs block up-conversion of the broad band input signal with the frequency of the local oscillator 5 being selected by the synthesiser 6 such that a desired channel is centered on a high first intermediate frequency. The synthesiser 6 and the synthesiser 11 are, for example, controlled via an 12C bus micro-controller (not shown).
The output of the first frequency changer 3 is filtered by the filter 7, which has a centre frequency at the predetermined first intermediate frequency and a pass-band characteristic such that the desired channel and a small number of further channels on either side of the desired channel are supplied to the second frequency changer 8.
The second frequency changer 8 performs a block down-conversion such that the desired channel is centered on the second much lower intermediate frequency. The output signal of the second frequency changer 8 is filtered by the filter 12 whose centre frequency is centered on the centre frequency of the desired channel following the second conversion and which has a pass-band for passing the desired channel and for rejecting or substantially attenuating the adjacent channels. The desired channel at the second intermediate frequency is amplified by the amplifier 13 and supplied to the tuner output 14.
Known types of tuner (of the type shown in FIG. 1 and of other types such as the single conversion type) are required to be able to cope with all extremes of signal conditions. In particular, such tuners are required to handle a wide dynamic range of signals and to cope with worst case composite signal loading, maximum channel to channel ripple and various other conditions, both singly and in any combination.
As a result of these requirements, for much of the time during operation, each tuner in a cable distribution system provides a much higher performance than is actually required by the individual channels being received. For example, in the case of modulated digital signals, there is no advantage in providing a tuner performance which exceeds a threshold value for signal to noise plus intermodulation performance. In such cases, the presence of complex Forward Error Correction techniques, such as Read Solomon Coding and Viterbi puncturing, means that there is no or no perceivable improvement in reception when a minimum threshold of signal to noise plus intermodulation is achieved.
The power consumption of a tuner is generally a direct function of the performance of the tuner. The term “direct function” in this context means that, for over at least a range of power consumptions, the performance of the tuner is a monotonic function of the power consumption of the tuner. In other words, the tuner performance increases monotonically as the tuner power consumption increases. Thus, more power is consumed, for example from the cable distribution network powering the modems 24, than is necessary to achieve the desired performance. Unnecessary power consumption is undesirable for many reasons, such as cost and environmental factors. With the increasing use of cable distribution systems and consequently of modems incorporating such tuners, the problem of increased power consumption is in turn increasing and is undesirable.
Known types of tuners, such as that illustrated in FIG. 1, are individually designed for different applications. For example, the reception of analogue signals requires very low intermodulation and cross-modulation generation but the phase noise requirement is not particularly demanding. On the other hand, reception of digital signals does not require such low intermodulation and cross-modulation generation but a better phase noise performance is required. Thus, individual tuners are specifically designed for different applications and this particularly affects the frequency changers. In particular, such tuners are designed in accordance with the modulation standard which they are required to receive. It has therefore not been possible to provide a “universal” tuner design which can be used for many different applications because achievement of the specification required to meet all standards would result in a non-optimum design having high power consumption and being commercially unattractive or unacceptable.
GB 2314706, GB 2290927, GB 2250402, EP 0795967, EP 0777334 and GB 2 317 080 disclose techniques for reducing power consumption in mobile radio equipment such as mobile cellular telephones. For example, GB 2 290 927 relating to mobile telephone applications uses the error rate from a demodulator to control the power supply of a radio receiver so that an adequate error rate performance is maintained. In the case of GB 2 250 402, the supply current to a receiver front end, comprising a radio frequency amplifier, a first frequency converter and a first intermediate frequency amplifier, is controlled so as to achieve a desired signal strength at the output of a second intermediate frequency stage. A similar technique is used in EP 0 795 967, in which the current consumption and hence the gain of a radio frequency amplifier are controlled.